What is currently the most widespread standard for local communication networks, so-called “local area networks” (LANs) is Ethernet. With Ethernet, data can currently be transmitted at a rate of up to 100 Mbit/sec (Mbps). LANs are local communication networks which are limited to a geographical area and are composed of one or more servers and workstations, so-called nodes, which are connected via a communication line network, e.g. a coaxial, fiber-optic or twisted pair cable. Various network topologies, e.g. bus, star or tree structures, are possible in the case of LANs.
LANs are operated by means of a network operating system and a network protocol. Ethernet constitutes such a network protocol. Furthermore, Ethernet supports a wide variety of communication protocols e.g. the TCP/IP protocol or the IPX protocol. In the OSI layer model, the international reference model for data transmission in a network, which is constructed from a layer stack comprising seven layers, a set of protocols being defined for each layer and in each case making their services available to the next higher layer, Ethernet is assigned to the second layer, the so-called data link layer. In this data link layer, the data to be communicated are bundled in packets to which specific items of information for the respective communication protocol are added. The data link layer is responsible in the network for the transport of the data packets from node to node and for error detection.
In the case of the Ethernet concept, the data link layer is subdivided into two levels, the first level adding to the data a so-called header, which contains items of information which are required for a correct data transmission by the receiver protocol. In the second level of the Ethernet protocol, the data packets are then encapsulated with the aid of an additional header and a further so-called trailer for the transport of the data packets from node to node. With such Ethernet data packets, so-called Ethernet messages, it is possible to transmit data with a length of up to 1500 bytes.
Ethernet furthermore defines the access method with regard to how the individual nodes are permitted to utilize and occupy the physical connecting paths of the network. In this case, Ethernet operates according to the so-called CSMA/CD (carrier sense multiple access/collision detect) access method. In the case of this access method, the node wishing to transmit checks prior to transmission whether the transmission path is free. The data are then sent. Since all nodes can send their data simultaneously, collisions may occur. The transmission operation is then interrupted by the node which notices the collision. In order to prevent two nodes from starting transmission in a manner offset by a short period of time, all transmitting nodes generate a so-called JAM signal in order that all nodes situated on the transmission path terminate the processing of the current data packet and thus do not disturb the transmission operation.
The Ethernet protocol is principally used in office communication networks. On account of the advantages of the Ethernet concept, in the use of standard hardware and software components and also the possibility of achieving high data transmission rates even with simple networking technology, there is a desire to be able to use the Ethernet network communication also in industrial production for data interchange and for carrying out control tasks.
However, in particular the deficient real-time capability of the Ethernet protocol has hitherto permitted only limited use in automation technology. This is because the control of machinery requires a cyclic processing of the control task to ensue without temporal fluctuations, i.e. with only small deviations from the desired cycle time in the region of a few microseconds, the system reacting to the regulating request with a predictable response time. However, the CSMA/CD access method used by Ethernet does not guarantee such fixed reaction times. This is because in the event of very high network capacity utilization, it can happen on account of the access method used that Ethernet messages cannot be transmitted over a certain time period, so that a guaranteed response time to a regulating request is not ensured.
In order nevertheless to be able to achieve real time to an extent in Ethernet networks, such Ethernet networks are often constructed in a star topology using so-called switches. In this case, each node, also referred to hereinafter as subscriber, has a point-to-point connection to the switch of the network. The switch examines each Ethernet message passing through on the network with regard to the address of the addressed subscriber and then forwards the Ethernet message to the addressed subscriber via the corresponding point-to-point connection. However, such a network topology made up entirely of point-to-point connections between the subscribers and the central switch requires complicated cabling and is therefore associated with high costs.
In order to ensure real-time capability and fast reaction times in Ethernet networks, there is furthermore the possibility of allowing the operation for transmission of the Ethernet messages to be controlled by superordinate protocol layers instead of by the CSMA/CD access method. However, the consequence of this is that the additional real-time protocol layer required gives rise to an intensified capacity utilization at the subscribers, which therefore require a powerful microcontroller, which in turn results in high costs.
Furthermore, the transmission of Ethernet data packets having lengths of up to 1500 bytes and high data transmission rates of 100 Mbit/sec requires that the Ethernet interfaces provided at the individual subscribers are upgraded with powerful data processing units in order to be able to store and rapidly read out the large data packets.
Particularly when control tasks require process data with only a small number of bytes, the mandatorily required data length of the Ethernet messages prevents a cost-effective interface design. This is because Ethernet, as a station-oriented network, requires data lengths of at least 100 bytes, which, at a high data transmission rate of 100 Mbit/sec, as explained, necessitates a powerful microcontroller for the transmission and reception operation. This holds true all the more if, in order to ensure a real-time capability, a real-time communication protocol is superposed on the CSMA/CD access method of the Ethernet protocol, so that in principle large Ethernet message packets already result on account of this additional protocol.